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constant::Atom
unique symbols

constant::Atom - unique symbols


NAME

constant::Atom - unique symbols


SYNOPSIS


        use constant::Atom qw (red yellow blue);

        

        my $color = red;

        

        print "Just as we thought!\n" if $color eq red;

        print "This will never happen.\n" if $color eq blue;

        print "Atoms never equal strings!\n" if $color eq 'red';

        

        print "Color is ".$color->name."\n";

        

        

        #The following raises an exception, because addition isn't defined for Atom objects.

        $color + 1;


DESCRIPTION

Unlike constants declared with constant, Atoms are not associated with any specific scalar value. Instead, Atoms have their own independent identity, and will only compare positively (via the 'eq' test) with other identical Atoms. All other operations on Atoms are undefined, including casting to a string and casting to a number.

Atoms are used in place of constants in situations where a unique value is needed to represent some idea or program state, but where that value is not naturally associated with a scalar value, and shouldn't be confused with one. Atoms are similar to C enums in this respect, except that Atoms do not have an ordinal value.

Below is an example of where an Atom would solve a problem:

#use constant::Atom 'error';
use constant 'error' => 999999;




        sub bar {

            my($arg) = @_;



            #Always return $arg for demonstration purposes (not error).

            return 1 ? $arg : error;

        }



        my $foo = bar(999999);

        print "Foo: $foo\n";



        print $foo eq error ? "Foo returned error." : "Foo returned $foo.";

Output: Foo returned error.

In the above example, the programmer is trying to choose some unlikely value to alias 'error' to. The problem is, if 'bar' is ever accidently called with this same value, the program will mistakenly believe that 'error' had been returned.

This doesn't happen with Atoms.


        use constant::Atom 'error';

#       use constant 'error' => 999999;

        

        sub bar {

            my($arg) = @_;

            

            #Always return $arg for demonstration purposes (not error).

            return 1 ? $arg : error;

        }

        

        my $foo = bar(999999);

        print "Foo: $foo\n";

        

        print $foo eq error ? "Foo returned error." : "Foo returned $foo.";

Output: Foo returned 999999.


COMPARISON TO ALTERNATIVES

An alternative to using an Atom is to use a constant aliased to a reference to an arbitrary scalar:


        use constant myconstant => \"";

There are two advantages of Atoms over this kind of constant


        - Scalar references can compare positively to numeric values:

                        use constant myconstant => \"";

                        my $numeric_value = myconstant + 0;

                        die "Trapped!" if $numeric_value == myconstant;

        

        - Atoms maintain their identity through serialization, even between processes:

                

                use constant::Atom 'myatom';

                use Storable qw (freeze thaw);

                print "Just as we thought!" if myatom eq thaw(freeze(myatom));


ATOMS AS STRINGS

An atom cast (stringified) into a tring produces a representation that may be useful for debugging purposes:




        use constant::Atom 'myatom';



        my $value = myatom;

        print "Myatom cast into a string: $value\n";

Output: Myatom cast into a string: Atom=SCALAR(0x18508dc)=main::myatom

Stringified Atoms can be used used as hash keys, matched to a regexps, etc. When this happens, the string value is not guarunteed to be unique. Although it is unlikely that you will ever accidently cast an Atom into a string, and even more unlikely that another string value will equal the string representation of the Atom, you might want to use constant::Atom::Strict to be 100% safe:


        use constant::Atom::Strict 'myatom';

        

        my $value = myatom;

        print "Myatom cast into a string: $value\n";

Output: Can't cast Atom::Strict object 'main::myatom' into a string. Use the 'fullname' method for a string representation of this object at C:\test7.pl line 5


OTHER METHODS

name

        package Languages;

        use constant::Atom qw (English);

        my $language = English;

        

        #Get the name of the constant

        print "Language: ".English->name."\n";

        

Output: English
fullname

        package Languages;

        use constant::Atom qw (English);

        my $language = English;

        

        #Get the string-representation of the constant, which is simply the fully-qualified symbol name.

        "$language" eq $language->fullname or die "These should be the same.";

        

Output: Languages::English




SEE ALSO

constant


AUTHOR

Jonathan R. Warden <john@newchester.com>


COPYRIGHT

Copyright 2004 Jonathan R. Warden. All rights reserved.

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

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